Newt's emerald by Garth Nix

Allen & Unwin, 2015. ISBN 9781760112653
(Age: 10+) Highly recommended. Fantasy, Historical fiction, Regency romance, Crime. A story that draws the reader in from the start, Nix mixes historical fiction with a bit of magic, and a touch of romance along with one funny situation upon another to create a Regency tale of Truthful Newington, a strong minded but naive young woman about to make her debut into London society. But her family's jewel, the Newington Emerald is stolen just as her father displays her inheritance, and she vows, along with her cousins, the Newington Lacy's to find it. But being a polite young woman she knows that she cannot ask the questions she wants to ask in the places in London she needs to go, so with the help of her great aunt, Lady Badgery she masquerades as her French cousin, Chevalier de Vienne. As he is about to join the monastery, no one will question his innocence and lack of background. So far so good, but when Truthful meets the acerbic Major Barnett, she sees that she needs him to help her. Unbeknown to her he has orchestrated their meeting as he and his masters question the motives of this particular  Frenchman in London.
Napoleon has just been captured and the theft of the emerald means a group will have all the money they need to repatriate Napoleon from the Rock of Gibraltar and begin again. The English government does not want this and along with General Ley is doing all they can to prevent it. But Truthful gets enmeshed in the plot, and after saving Barnett's life is expected to retire back to her gracious surroundings and prepare for balls. This is now beyond her and hearing that her enemy, Lady Plathened, a strong sorceress who has her emerald will be at the same masquerade ball in Brighton as she, dons again her own disguise to help Barnett is his capture. Fun from start to finish, Truthful finds herself in many hair raising scrapes where her quick wits and minimal training in magical powers are needed. The Regency background is well delineated, the characters engaging and the storyline fascinating. I loved it and hope to read more about this young woman.
Fran Knight